1. Field of Invention
This invention relates to magnet assemblies and particularly assemblies which are useful for inducing magnetic fields within vacuum chambers.
2. Background of Invention
In the fabrication of thin film magnetic devices—sensors, magnet heads, MRAM magnetic materials are often deposited by sputtering. During such processes, it is also, frequently, a requirement that a magnetic field is applied to the substrate so that the resulting films are magnetically aligned. It will be appreciated, therefore, that if the magnetic field across the substrate is not completely aligned, problems of non-uniformity will arise.
Considerable thought therefore has been given to the design of magnetic field generators for use in connection with vacuum deposition systems. Although either permanent magnets or electromagnetics can be used, the latter are preferred, because they can be switched on or off or otherwise modulated. Typical field strengths are in the range of 4 to 8 kA.m−1. For thin film applications, the component of the magnetic field perpendicular to the growing film is generally of no concern. However alignment of the in plane film needs to be typically no more than +/−2° over the area of the substrate as in plane non-uniformity is often required to be less than +/−5%.
Whilst large coils placed externally on opposite sides of the vacuum chamber, might well provide the necessary configuration, they are very bulky and require high power and only provide a magnetic field in a single plane or direction.
It is generally preferred to mount the field generator in close proximity to the substrate support. The approaches generally fall into two types. The first has two opposing pole pieces, connected by a low reluctance magnetic circuit incorporating one or more coils, which can be located beneath or adjacent to the substrate support. The disadvantage of this approach is that the pole pieces must lie in, or very close, to the plane of the substrate. They must also extend laterally well beyond the substrate in order to project an acceptably uniaxial field across the substrate. They thus encompass a relatively large rectangular space surrounding the substrate and this can interfere with other hardware within the process environment. Such a configuration is shown in FIG. 1.
A development of this approach is shown in U.S. Pat. No. 6,545,580. It will be noted that in order to achieve acceptable field alignment, additional L-shaped “pole pieces” are required. Since these must be located at or close to the plane of the substrate, much of the advantage of the upward projection is lost.
The second approach is illustrated in U.S. Pat. Nos. 5,630,916 and U.S. Pat. No. 6,042,707, which aim to project a field from below the substrate support. They utilise a generally planar coil—i.e. a coil wound around a soft magnetic core in the form of a flat plate. The weakness of the planar coil approach, as described in U.S. Pat. No. 5,630,916, is illustrated in FIG. 2, which is that the coil must extend significantly beyond the edge of the substrate, if uniformity over the substrate is to be maintained. In U.S. Pat. No. 5,630,916 it is stated that the coil should be at least 50% larger than the substrate in order to achieve acceptable field uniformity.